....so I made two square waves - 40hz and 41hz- in audacity and played them both at the same time and it has this weird effect. what is this? is this phasing(if not, can somebody explain phasing to me?)?

Bingo. Sometimes guitar players will do this by playing the same high pitched harmonic on two different strings at the same time and then dive bombing slowly with the tremolo. The two harmonics go down in pitch at different rates and as the frequencies diverge it creates a building dissonance.

From a physics point of view, what you heard is called a Beat and is caused by alternating constructive and destructive interference between the two waves. So even when you're not drumming you're creating beats... how cool is that. Anyway, here's a detailed explanation.

The closer the two frequencies are, the more distinguishable the beat characteristics. Polly's sound file is a pretty good display of this phenomenon. For two sine waves with frequencies f1 and f2, you will get a wave with frequency (f1+f2)/2, so the average of the frequencies. The amplitude of this wave will be modulated with frequency (f1-f2)/s, so half the difference in frequency. For 40 and 41Hz you'd get a tone of 40.5Hz modulated in amplitude with a frequency of 0.5Hz in the case of sine waves. Rectangular waves are a different story as the waves themselves are a big series of sine waves summed together to create a rectangular wave (see the Fourier Series for more information on this - it's actually pretty cool.

The closer the two frequencies are, the more distinguishable the beat characteristics. Polly's sound file is a pretty good display of this phenomenon. For two sine waves with frequencies f1 and f2, you will get a wave with frequency (f1+f2)/2, so the average of the frequencies. The amplitude of this wave will be modulated with frequency (f1-f2)/s, so half the difference in frequency. For 40 and 41Hz you'd get a tone of 40.5Hz modulated in amplitude with a frequency of 0.5Hz in the case of sine waves. Rectangular waves are a different story as the waves themselves are a big series of sine waves summed together to create a rectangular wave (see the Fourier Series for more information on this - it's actually pretty cool.

Mr Spock called and said he wants his brain back.

To the O/P...uh...yeah...I was going to say that too, but Big Philly beat me to it.

It's actually quite elementary physics / mathematics (the beat phenomenon - Fourier Series are advanced calculus material). Not even Spock-ish by a long shot.
I hold a Bachelor of Science degree, my BSc assignment was in the field of acoustics so I can consider myself knowledgeable on wave phenomena :)